1. Field of the Invention
The present invention relates to hearing aid of the type having an oscillation detector and to a method for detecting oscillations, and thus feedback, in a hearing aid.
2. Description of the Prior Art
Methods for detecting oscillations and thus acoustic feedback in a hearing aid are known. As a rule, these methods are based on relatively complex algorithms or intensive frequency analyses (e.g. by means of Fourier transformation). This type of method of oscillation detection, as is known from European Application 0 656 737, for example, is relatively expensive in terms of hardware and is thus suitable for use in a hearing aid only in certain circumstances.
PCT Application WO 96/35314 teaches feedback suppression in a hearing aid in which the occurrence of feedback is detected. When feedback is discovered, the amplification is reduced in at least one frequency band and is left alone in all other frequency bands. To set the hearing aid definitively, a set of corresponding parameters is ultimately generated and stored. A disadvantage of this feedback suppression is that the occurrence of feedbacks is detected only during adjustment of the hearing aid, and feedback during the normal operation of the hearing aid are not detected and eliminated.
German OS 37 33 983 teaches a method for damping noise in contrast to speech, which also reduces the tendency for feedback. In this method the spectral distribution of detected sound signals is calculated by means of Fourier analysis in a number of frequency windows and is compared to predetermined limit values. The feedback suppression is based on a frequency transposition of the microphone signal, so that feedback does not even arise. This method, however, leads to a distortion of the input signal and thus impairs the sound quality of the hearing device.
German PS 39 27 765 teaches a hearing device with signal processing for improving the separation of voice signals relative to noise signals. Low-frequency noise signals are detected and damped. Acoustic feedback is not detected and suppressed.
The article “Reducing Acoustic Feedback in Hearing Aids”, Maxwell et al. (IEEE Transaction on Speech and Audio Processing, Vol. 3, No. 4, July 1995), an adaptive filter is used, whose frequency response always corresponds to the inverse of the signal frequency response. When the input signal contains strong tonal components, a type of blocking filter thus develops, whose blocking band is located at the strongest (highest peak) frequency component of the microphone signal. When the energy of the microphone signal in the blocking range exceeds a threshold value, the output signal is the output signal of the blocking filter. Otherwise the microphone signal remains unchanged. A reliable feedback detection does not take place. This signal processing not only reduces feedback, but also all dominant spectral components of the input signal generally. Thus speech signals, which have a distinctive frequency structure, are also altered. This leads to a considerable impairment of the sound quality, as Maxwell et al. report.
Lastly, German PS 38 02 903 teaches a hearing device with a parallel circuit having several frequency selection channels. To suppress noise signals in regions in which these noise signals are not hidden by spectral components of speech, threshold switches with a control signal generator are located in each of these channels. The sum of the output signals of the Individual threshold switches is fed to the acoustic output converter. This circuit arrangement is also not suitable for suppressing feedback.